Problem: In history class, the girl to boy ratio is $9$ to $9$. If there are a total of $54$ students, how many girls are there?
A ratio of $9$ girls to $9$ boys means that a set of $18$ students will have $9$ girls and $9$ boys. A class of $54$ students has $3$ sets of $18$ students. Because we know that there are $9$ girls in each set of $18$ students, the class must have $3$ groups of $9$ girls each. There is a total of $27$ girls in history class.